The prevent invention relates to industrial chemistry and more particularly to a method for the largest recovery of ethylene and propylene from gases produced by the pyrolysis of hydrocarbons. This invention relates also to an apparatus for carrying out said method on an industrial scale.
The production of light olefines, such as ethylene and propylene, has considerable economic importance. They are basic chemical products of the polymer industry which are transformed to intermediate products widely used in the consumer goods industry, the motor and mechanical industry, in building, and in domestic articles a.s.o.
The world production of ethylene is around 60 million tons yearly, about 25% of which is produced and used in Europe.
A plant for the production of ethylene needs a considerable investment. In 1990, it was about 1 billion lire per ton per year of ethylene. The energy cost is relatively high, i.e., around 15-20% of the technical production cost.
It is evident that any improvement in the technological design of the plant allowing the investment cost and/or the energy consumption to be reduced has a noticeable industrial interest.
The high operating reliability is, of course, another significant aspect because of the direct influence on the production cost. Both an increase of the operating reliability and a reduction of the maintenance cost, with the consequent reduction of the investment, correspond to the reduction of the number of equipment and, above all, the machinery requested for the production.
The industrial production of the olefines is presently based upon the pyrolysis of hydrocarbons or oil gas at high temperature and in the presence of steam.
A typical volume percentage composition of the dry product gas is the following:
______________________________________ H.sub.2 CH.sub.1 C.sub.2- C.sub.3- C.sub.4+ Ethane Propane Total ______________________________________ 30 15 30 10 5 8 2 100 ______________________________________
The recovery rate and the purity requested for the produced olefines are both very high. The recovery of the ethylene is 99.5% with a purity of 99.95%. The recoverty of propylene is 98% with a purity of 99.5%.
The industrial recovery and the depuration method is based upon the fractional condensation of the pyrolysis gas components and their subsequent condensation by distillation carried out in a sequence of single operations, as shown in the following block diagrams A, B, C: ##STR1##
The present state of the industrial practice consists in processing gas in a first section of the system intended for recovering ethylene and components having the highest boiling point which are then separated from hydrogen and methane.
In the next section, the above mentioned mixture is then split into two fractions, one of which contains ethylene, ethane and acetylene, and the other contains propylene and other heavier hydrocarbons.
The first section typically includes two to five distillation columns provided with heat exchangers, vessels and pumps. The second section generally includes one distillation column with the associated equipment. Thus, the whole assembly is a very complex industrial plant.
The present invention relates to the section for the recovery of ethylene and propylene from the raw gas by splitting the mixture containing them in two fractions: the first fraction containing almost all the produced ethylene (with a recovery rate of more than 99.5%); the second fraction containing almost all the propylene (with a recovery rate of more than 99%) and all components having the highest number of carbon atoms (i.e. butadiene, aromatic compounds and gasolines).
This invention seeks to provide a method and an apparatus enabling a considerable improvement of the energy balance and a significant reduction of the number of equipment necessary for the separation as shown in the following diagram D relating to the ethane cracking. ##STR2##
As can be seen from such diagram, the conventional linear open sequence of the single operations is replaced by a triangular or cyclic sequence of single operations at the center of the system.
This invention will now be described with reference to the accompanying drawing showing by way of an illustrative, non-limitative example, a preferred embodiment of the invention.